This invention relates to apparatus for decontaminating a radioactively contaminated coolant, and more particularly it is concerned with an apparatus for decontaminating a radioactively contaminated coolant which circulates through the pressure vessel of a nuclear reactor of the boiling-water type nuclear power plant.
Boiling-water type nuclear power plants of the prior art generally comprise a cooling water decontaminating system for removing a radioactively contaminated ionizing metal corrosion product from cooling water flowing through the pressure vessel during operation of the reactor, and a residual heat removing system for removing, when the reactor is shut down, residual heat from the cooling water which has been heated by decay heat in the pressure vessel.
In detail, in a boiling-water type nuclear power plant, the cooling water flowing through the core of the pressure vessel is heated by decay heat of nuclear fuel in the core and produces steam which is led through a main steam line to a steam turbine where energy goes into the turbine. After giving off the energy, the steam is passed through a condenser where it is changed back into water. Recycling of the cooling water for supplying the cooling water to the core is effected by means of recycling pipe lines and a recycling pump.
The cooling water decontaminating system is constructed as follows. A portion of the cooling water of elevated temperature containing a radioactively contaminated metal corrosion product which is recycled through the recycling pipe lines when the nuclear reactor is in operation is withdrawn by means of a pump, and passed on to an ion exchanger through a heat recovery exchanger and a cooler cooled by raw water (river water). The radioactive metal corrosion product in this portion of cooling water is removed therefrom by ion exchanger. The decontaminated cooling water is sent to the heat recovery exchanger where it is subjected to heat exchange with another portion of cooling water of elevated temperature supplied thereto from the core. After being reheated in this way, the decontaminated cooling water is returned to the core of the pressure vessel.
On the other hand, the residual heat removing system which uses some pipe lines in common with a safety system for the reactor is constructed as presently to be described. Upon the nuclear reactor being shut down, the recycling pump stops its operation, so that recycling of the cooling water is stopped. At this time, a portion of the cooling water in the recycling pipe lines is delivered, by means of a pump in a coolant withdrawing pipe line, to a cooler where it is cooled by a refrigerant and returned to the core through a return pipe line. The coolant withdrawing pipe line and return pipe line each mount therein a valve, the valves being closed when it is not necessary to remove residual heat from the core, such as while the reactor is in operation. At this time, an anticorrosive agent is incorporated in the body of water in this closed system.
If the body of water in the closed system which contains an anticorrosive agent is returned as it is to the core for removing residual heat when the reactor is shut down, the anticorrosive agent will undergo thermal decomposition in the core and have some detrimental effects, such as corrosion, on the core. This is not desirable. Thus, there is the need to effect flushing prior to removing residual heat while keeping the closed system in a condition in which it is isolated from the core.
It is only after a flushing operation is performed that removal of residual heat is carried out. A flushing operation consists in washing with clean water the valves in the coolant withdrawing pipe line and return pipe line of the closed system several times. Then the anticorrosive containing cooling water is stored in a vessel made of a radiation shielding material.
We have made a discovery that, if the operation of the recycling pump is stopped, a radioactively contaminated metal corrosion product (hereinafter referred to as clad) which has hitherto been adhering to the core separates itself from the core and is incorporated in the cooling water in the pressure vessel, with the result that the concentration of the clad in the cooling water in the core becomes twenty to thirty times as high as that during operation of the reactor. Before this discovery was made, it was only the cooling water decontaminating system that was covered with a radiation shielding member, and the residual heat removing system was disposed outside the containment vessel because the possibility of incorporation of the clad in this system was not expected.
The aforementioned discovery has raised the following problems:
(1) Passing of the clad onto the residual heat removing system increases the region which is radioactively contaminated; PA1 (2) The practice of performing decontamination of the cooling water only during operation of the reactor, which has hitherto been considered to achieve satisfactory results, cannot achieve the effect of removing the clad because no clad is incorporated in the cooling water during operation of the reactor; PA1 (3) Installing the residual heat removing system in addition to the cooling water decontaminating system increases capital cost; and
(4) The operation of removing residual heat should be performed after flushing of the pipe lines of the closed system is effected. This increases the reactor shutdown time by as much time as required for effecting flushing, thereby lowering the working rate of a reactor power plant.